When you add someone to your group that does not yet have a Prempoint account, when they sign up the group will automatically be made their active group.
However, if you add someone that already has a Prempoint account, they will be added to the group but it will NOT automatically be made their active group since they could be part of multiple groups. In this case, they will have to make your group active in their account. This can be done in the web console or the Prempoint mobile app.
In the web console, you can change your active group using the dropdown at the top right of the page:
In the Prempoint mobile app, you can change your active group from the dropdown in the middle of the "Manage" screen: